Div background image wont show on mobiles - html

Question

I have a div class with a background image set. It displays ok on pc browsers but it wont show the image on mobile browser(i have tried android and ios)
the html
<div class="wrapper">
<div class="headerdiv">
<?php include '../inc/header.php';?>
</div>
<div id="wherewhen">
<div id="when">
<span class="wherewhen">WHEN IS IT?</span><br />
<span class="wherewhentext">9TH/10TH FEBUARY 2014</span>
</div>
<div id="where">
<span class="wherewhen">WHERE IS IT?</span><br />
<span class="wherewhentext">HOLIDAY INN, MILTON KEYNES, ENGLAND</span>
</div>
<div id="what">
<span class="wherewhen">WHAT IS IT?</span>
</div>
<div id="info">
<span class="wherewhentext">
A BACK IN THE DAY STYLE CONVENTION
FOR DISENFRANCHISED ARTISTS AND TATTOO FANS FED UP
WITH TODAY'S COMMERCIAL CORPORATE EXPOS.
</span>
</div>
</div>
<div class="poster">
</div>
the css
html, body{
margin:0;
padding:0;
width:100%;
height:100%;
overflow:auto;
background:#000000;
}
.wrapper{
width:80em;
margin: 0 auto 0 auto;
background:#000000;
overflow:hidden;
}
.poster{
float:left;
margin:-900px 0 0 910px;
width:370px;
min-height:600px;
background:#ffffff url('../images/siteimages/background.png');
}
#wherewhen{
background:#392da3;
width:600px;
margin:20px 20px 0 295px;
text-shadow: 2px -2px #000000;
}
#where{
float:left;
text-align:center;
width:20em;
}
#when{
float:right;
text-align:center;
width:15em;
}
#what{
width:20em;
text-align:center;
margin:0 auto 0 auto;
}
#info{
width:30em;
margin:0 auto 0 auto;
}
Can anyone offer assistance here?
Thanks...

Answer 1

oki, #poster background could be in #wrapper with background-attachement:fixed.
Else from your code (#poster outside #wrapper )
#wrapper {
position:relative;
z-index:1;
/* your rules */
}
#poster {
position:fixed;
z-index:0;
top:0;
left:0;
/* your rules */
}